The present invention relates to a liquid crystal apparatus for effecting an active matrix drive by using a liquid crystal having a spontaneous polarization.
At present, most of liquid crystal display apparatus for use in monitors of liquid crystal television sets, word processors and personal computers principally employ a TN (twisted nematic) mode or an STN (super twisted nematic) mode using a nematic liquid crystal as a display mode.
In the case of using such a TN or STN mode for multiplex driving scheme, however, an increase in the number of scanning signal lines is liable to lower a contrast. Even if a drive waveform is optimized in order to provide practical display qualities, the number of scanning signal line has been restricted to ca. 400-500 lines at best.
In order to sole such a problem that display qualities are lowered with an increased number of scanning signal lines in a liquid crystal display mode (TN or STN mode), there has been proposed an active matrix (display) mode using a plurality of switching devices or devices, such as MIM (metal-insulator-metal) devices or TFTs (thin-film-transistors), disposed in a matrix form on a substrate.
In this case, however, a nematic liquid crystal used as a liquid crystal material therefor shows a slow response speed of several hundred msec particularly for gradation display signals, thus failing to follow high-speed motion display. As a result, it is difficult to provide sufficient display qualities. Further, in the above-mentioned TN (or STN) mode, liquid crystal molecules cause switching between a state where they are twisted and in parallel with a substrate and a state where they are perpendicular to the substrate, thus resulting in a large viewing angle-dependence based on its principle.
On the other hand, there have been developed display devices using a liquid crystal having a spontaneous polarization, such as a ferroelectric or chiral smectic liquid crystal, in view of, e.g., a higher-speed responsiveness and a wider viewing-angle characteristic when compared with those of the TN or (STN) mode using the nematic liquid crystal.
For example, as a display device using a liquid crystal showing ferroelectricity, there has been proposed a surface-stabilized ferroelectric liquid crystal display device as described in Japanese Laid-Open Patent Application (JP-A) 56-107216, wherein multiplex driving scheme is practiced according to a simple matrix mode utilizing bistability of liquid crystal molecules. However, this driving scheme fails to continuously change a resultant transmittance since it performs two-value (binary) driving using bistable states of liquid crystal molecules, thus not facilitating gradational display. For this reason, there have been proposed various gradational display methods using, e.g., pixel division, time-division display, and image processing.
Further, there have been proposed active matrix driving schemes utilizing a high-speed responsiveness and a wide view-angle characteristic of a ferroelectric or antiferroelectric liquid crystal. For example JP-A 5-100208 discloses a method of effecting gradational display by performing active matrix driving of an antiferroelectric liquid crystal assuming three stable states and JP-A 9-68728 discloses a gradational display method using an active matrix driving scheme and a thresholdless-antiferroelectric liquid crystal (TL-AFLC).
However, in the case of the active matrix driving scheme using a chiral smectic liquid crystal (e.g., a ferroelectric or antiferroelectric liquid crystal), an effective voltage applied to the liquid crystal is substantially lowered to cause image-quality deterioration as described in, e.g., (1) A full-color thresholdless Antiferroelectric LCD exhibiting wide viewing angle with fast response time, T. Yoshida et al., SID (Society for Information Display) 97 DIGEST, pp. 841-844, and (2) Voltage-holding properties of thresholdless Antiferroelectric liquid crystals driven by active matrices, T. Saishu et al., SID 96 DIGEST, pp. 703-706. More specifically, in the case where an antiferroelectric (or ferroelectric) liquid crystal having a spontaneous polarization is driven (i.e., subjected to switching) by using an active element or device (e.g., TFT), an inversion of the spontaneous polarization of the liquid crystal causes a lowering in holding voltage to substantially decrease a voltage applied to the liquid crystal, thus resulting in a deterioration in image qualities, such as a low contrast.
The lowering in holding voltage leading to inferior image qualities will be simply described hereinbelow with reference to FIGS. 3-5.
FIG. 4 shows an equivalent circuit of one pixel portion of a liquid crystal device using a liquid crystal having a spontaneous polarization (in this instance, the TL-AFLC as described in the above document (1) is used), and FIG. 3 shows a V-T (voltage-transmittance) curve as an optical response characteristic of the liquid crystal device using the TL-AFLC when a low-frequency triangular waveform is applied.
Referring to FIG. 4, the equivalent circuit includes a TFT 14, a liquid crystal capacitance (Clc) 31, a storage capacitance (Cs) 32, a spontaneous polarization of the liquid crystal (Ps) 50, a storage capacitance electrode 30 and a common electrode 42. The TFT 14 includes a gate electrode, a source electrode and a drain electrode and supplies a voltage to the liquid crystal through the drain electrode. The storage capacitance (Cs) 32 for holding a voltage applied to the liquid crystal layer at the time of xe2x80x9cOFFxe2x80x9d state of the TFT 14 is disposed in parallel with the liquid crystal capacitance (Clc) 31.
FIGS. 5A-5D show drive waveforms applied to the pixel (shown in FIG. 4) and an optical response (transmittance) of the liquid crystal. More specifically, FIG. 5A shows a voltage waveform of a scanning selection signal applied to a scanning signal line (gate line) connected to the gate electrode of the TFT 14. FIG. 5B shows a data signal voltage waveform applied to a data signal line (source line) connected to the source electrode of the TFT 14. FIG. 5C shows a voltage waveform applied to the liquid crystal layer (portion) of the pixel concerned. FIG. 5D shows a transmittance of the pixel.
Referring to FIG. 5A, a gate voltage Vg as a signal for placing the gate of the TFT 14 in an xe2x80x9cONxe2x80x9d state is periodically applied in every selection period Ton. In synchronism with the gate voltage Vg, a source voltage (data signal voltage) Vs is applied to the source electrode, thus being supplied to the liquid crystal layer via the drain electrode of the TFT 14 (FIG. 5B). The source voltage Vs has a polarity which is inverted periodically in order to prevent a deterioration of the liquid crystal due to an asymmetrical voltage applied to the liquid crystal layer. Referring to FIG. 5C, a voltage Vpix applied to the liquid crystal layer is applied in a selection period Ton so that the Vpix has a polarity which is opposite to that in a period immediately before the selection period Ton. The liquid crystal starts to transfer its alignment state to that depending on the polarity of the voltage Vpix applied to the liquid crystal layer in the selection period Ton. If the response time of the liquid crystal is sufficiently shorter than the Ton, the transfer of the liquid crystal is continued to a (subsequent) non-selection period Toff by a voltage based on a storage capacitance. The liquid crystal used has a spontaneous polarization, so that a voltage decrease (lowering) (corresponding to xcex94V) is caused by the spontaneous polarization when the direction thereof is inverted (FIG. 5C). As a result, the liquid crystal is finally placed in an alignment state corresponding to the voltage Vpix including the voltage drop xcex94V. Accordingly, as shown in FIG. 5D, an optical response of the liquid crystal is also continued to the non-selection period Toff.
As described above, in order to obtain a desired optical (display) state, it is necessary to effect a drive in view of the voltage drop due to the spontaneous polarization. The voltage drop phenomenon is, however, affected by the spontaneous polarization of the liquid crystal, driving voltage, storage capacitance, liquid crystal capacitance, etc., thus leading to such a problem that a desired gradational data is not accurately displayed.
A principal object of the present invention is to provide a liquid crystal apparatus having solved the above problems caused by the voltage decrease based on inversion of a spontaneous polarization of a liquid crystal.
A specific object of the present invention is to provide a liquid crystal apparatus capable of improving image qualities, particularly for gradational display, such as a contrast, while retaining a high-speed responsiveness at the time of effecting display by driving a liquid crystal device using a liquid crystal having a spontaneous polarization according to an active matrix driving scheme.
According to the present invention, there is provided a liquid crystal apparatus, comprising:
a liquid crystal device including an active matrix substrate, a counter substrate disposed opposite thereto, and a liquid crystal disposed between the active matrix substrate and the counter substrate; said active matrix substrate having thereon a plurality of scanning signal lines, a plurality of data signal lines intersecting the scanning signal lines, a plurality of switching devices each disposed at an intersection of the scanning signal lines and the data signal lines and connected to an associated one of the scanning signal lines, and a plurality of pixel electrodes each connected via one of the switching devices to an associated one of the data signal lines and form a pixel together with the liquid crystal thereat for applying a data signal voltage to the liquid crystal at the pixel; said liquid crystal having a spontaneous polarization and causing a state change accompanied with a polarity inversion thereof within a response time, and
drive means for sequentially selecting the scanning signal lines each in a scanning selection period and applying data signal voltages to the pixels along an associated scanning signal line, wherein the scanning selection period for a scanning signal line is shorter than the response time for the liquid crystal at a pixel on the scanning signal line thus being liable to leave a remaining portion of polarity inversion to reach a desired state change, and the data signal voltage applied to the pixel is set to include a compensation voltage for compensating for a voltage decrease caused by the remaining portion of polarity inversion.
According to the present invention, there is also provided a liquid crystal apparatus, comprising:
a liquid crystal device including an active matrix substrate, a counter substrate disposed opposite thereto, and a liquid crystal disposed between the active matrix substrate and the counter substrate; said active matrix substrate having thereon a plurality of scanning signal lines, a plurality of data signal lines intersecting the scanning signal lines, a plurality of switching devices each disposed at an intersection of the scanning signal lines and the data signal lines and connected to an associated one of the scanning signal lines, and a plurality of pixel electrodes each connected via one of the switching devices to an associated one of the data signal lines and form a pixel together with the liquid crystal thereat for applying a data signal voltage to the liquid crystal at the pixel; each pixel being provided with a storage capacitance disposed in parallel with the liquid crystal, and said liquid crystal having a spontaneous polarization and causing a state change accompanied with a polarity inversion thereof within a response time, and
drive means for sequentially selecting the scanning signal lines each in a scanning selection period and applying data signal voltages to the pixels along an associated scanning signal line, wherein the scanning selection period for a scanning signal line is shorter than the response time for the liquid crystal at a pixel on the scanning signal line thus being liable to leave a remaining portion of polarity inversion to reach a desired state change, and the liquid crystal device and the drive means are set to satisfy the following conditions:
Vs2xe2x89xa7{xcex94Qxc3x97M/(Clc+Cs)}+Vs1xe2x80x83xe2x80x83(12)
wherein Vs2 is a data signal voltage (volt) applied to one pixel, Vs1 is a voltage (volt) for providing writing data for the pixel based on a voltage-transmittance characteristic of the liquid crystal, xcex94Q is an amount (C) of inversion of the spontaneous polarization of the liquid crystal, Clc is a liquid crystal capacitance (F) at one pixel, Cs is a storage capacitance (F) at one pixel, and M is a proportion of the remaining portion of polarity inversion in a scanning selection period for one scanning signal line.
According to the present invention, there is further provided a liquid crystal apparatus, comprising:
a liquid crystal device including an active matrix substrate, a counter substrate disposed opposite thereto, and a liquid crystal disposed between the active matrix substrate and the counter substrate; said active matrix substrate having thereon a plurality of scanning signal lines, a plurality of data signal lines intersecting the scanning signal lines, a plurality of switching devices each disposed at an intersection of the scanning signal lines and the data signal lines and connected to an associated one of the scanning signal lines, and a plurality of pixel electrodes each connected via one of the switching devices to an associated one of the data signal lines and form a pixel together with the liquid crystal thereat for applying a data signal voltage to the liquid crystal at the pixel; each pixel being provided with a storage capacitance disposed in parallel with the liquid crystal, and said liquid crystal having a spontaneous polarization and causing a state change accompanied with a polarity inversion thereof within a response time, and
drive means for sequentially selecting the scanning signal lines each in a scanning selection period and applying data signal voltages to the pixels along an associated scanning signal line, wherein the scanning selection period for a scanning signal line is shorter than the response time for the liquid crystal at a pixel on the scanning signal line thus being liable to leave a remaining portion of polarity inversion to reach a desired state change, and the liquid crystal device and the drive means are set to satisfy the following conditions:
1/(nxe2x88x921) greater than A(2Psxc3x97S)/V0(Clc+Cs)ave,
wherein n represents the number of gradational levels per one period; A is represented by the following equation:
A={(Clc+Cs)maxxe2x88x92(Clc+Cs)min}/(Clc+Cs)ave,
where (Clc+Cs)max represents a maximum of the sum (Clc+Cs) of a liquid crystal capacitance Clc (F) at one pixel and a storage capacitance Cs (F) at one pixel, (Clc+Cs)min represents a minimum of (Clc+Cs) and (Clc+Cs)ave represents an average of (Clc+Cs); Ps represents a spontaneous polarization (C/cm2) per unit area of the liquid crystal; S represents a pixel electrode area (cm2) at one pixel; and V0 represents a saturation voltage (volt) for the liquid crystal providing a maximum transmittance.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.